Our experiences with the use of radio-active mercury brain scanning in the diagnosis of a series of intracranial tumors and granulomas have been discussed. It is our belief that this is the most valuable single test available today for demonstrating and localizing an intracranial space occupying lesion. It was proved of value in a high percentage of cases (83.1%). Not all tumors are demonstrable by this method. Glioblastomas and meningiomas gave diagnostic scans in the highest percentage of cases. Astrocytomas may not be demonstrated by this method. Epidermoids have not been shown with this technic in our experience. Tumors in the posterior fossa and at the base of the skull may be obscured by the radio-activity displayed by the muscles and the nasal mucosa at the base of the skull. The method is safe, is free from discomfort to the patient, is simple to perform and relatively inexpensive. It must not be used by the non-neuro-logically oriented physician to screen out those patients suffering from brain tumor. Unfortunately the tumors which this technic does not disclore are often the early tumors and those most benign, and thus those most amenable to prompt surgical intervention.